Undersea cables could detect tsunami signals

Tsunamis may be detectable with underwater fibre-optic cables, according to a new detailed model of the electrical fields the moving water generates.

The charged particles in the ocean water interact with Earth's magnetic field to induce voltage of up to 500 millivolts in the cables that ferry internet traffic around. With relatively simple technology, those voltage spikes could serve as a tsunami-warning system for nations that can't afford large arrays of other types of sensors.

"What we argue is that this is such a simple system to set up and start measuring," said Manoj Nair, a geomagnetist with the National Oceanic and Atmospheric Administration who led the research. "We have a system of submarine cables already existing. The only thing we probably need is a voltmeter, in theory."

The salt in ocean water makes it a good electrical conductor. Positively charged sodium and negatively charged chlorine ions in the solution are free to move. In a large movement of ocean water, these ions are carried across the Earth's magnetic field creating an electrical field.

Decades ago, Bell Labs researchers revealed that the movement of ocean water after the 1992 Cape Mendocino earthquake created "a large-scale motional electric field" that was detectable by an underwater cable. But the work wasn't followed up because alternative technologies were available that could take better measurements.

Rich countries like the United States can install sea bottom pressure arrays like those used by the Pacific Tsunami Warning Center. These directly detect the motion of large amounts of water.

But some countries can't afford to install and maintain those arrays, so it could be critical to have a lower-cost alternative.

Nair's work, which will be published in February's Earth, Planets and Space, quantified the physics of this lower-cost alternative by building a model of the catastrophic Indian Ocean tsunami of 2004. He and his team showed that the voltages induced in the submarine cables would be large enough to measure.

It's a major step towards turning this speculative idea into a real system, and he stressed that other groups would have to confirm the results of their model through observations.

"We treat this as a novel idea that we're putting forth, but it still needs to be taken seriously and verified by other groups," Nair cautioned.